In optical networks, laser sources are typically utilized to generate carrier signals on which information (e.g., data, voice, video) is impressed by some suitable modulation scheme. In wavelength division multiplexed (WDM) systems a plurality of lasers, or a broadband tunable laser, generates a plurality of carriers at different wavelengths corresponding to the different channels of the system. As optical networks become more prevalent and sophisticated, particularly in multi-access networks, there will likely be a need for more functionality in the laser sources. As service demands grow, technologies will be needed to increase the capacity of these networks to accommodate a larger number of users. Additional signal routing capability on the physical layer may be required to provide additional networking capabilities.
The prior art has endeavored to increase the capacity of optical WDM networks by resorting to SCM. Most have proposed frequency modulation subcarrier schemes known as FM-SCM. See, for example, W. I. Wang et al., Electronics Letters, Vol. 26, No. 2, pp. 139-142 (1990), N. K. Shankaranarayanan et al., J. Lightwave Technology, Vol. 9, No. 7, pp. 931-943 (1991), and M. Shell et al., IEEE Photonics Technology Letters, Vol. 8, No. 10, pp. 1391-1393 (1996). In the context of a CATV distribution network, an amplitude modulation subcarrier scheme known as AM-SCM has also been proposed. See, T. Uno et al., OFC '94 Technical Digest, Paper WM2, pp. 153-155 (1994).
Most frequently these prior art schemes have used multiple DFB lasers to generate the different WDM channel carriers. In some cases these lasers were directly modulated at RF rates to impose the FM or AM subcarrier channels on the optical carrier of each WDM channel. A limitation of directly modulated DFB lasers for FM-SCM is the resulting residual AM that is incurred. Therefore, a need remains in the art for a broadband wavelength tunable laser that enables WDM, AM-SCM and FM-SCM to be realized in a single device. This need is not satisfied, for example, in prior art DBR lasers and sampled grating lasers which do not provide broadband, continuous tuning around the center wavelength of each channel.